I had a P7 for a few years (now have an RP8). I think it's about the most underrated deck that Rega made, probably because no-one cared for the tooth-like ceramic blobs. However I always thought it was a bit disappointing when Rega abandoned ceramic on the 2nd tier turntables. The RP8 is better, mainly because of the plinth and the arm - the RB808 (now the 880 I think) is certainly better than the RB700, but that is a good arm anyway. The more recent cabling is very much improved.
As has been said, fitting a non-Rega arm to a Rega deck is pretty pointless. One thing you can do relatively inexpensively, however, is upgrade the PSU - the P7 came with the old TTPSU mk1. The Neo is in a different league and will also allow fine-tuning of the speed. I'd love to hear my old P7 with a Neo - I also had a Dynavector 17D3 on it which made for a very dynamic sound. That would be preferable in some ways to an SME....